"We can't be breeding right now," says Les Knight. "It's obvious that the intentional creation of another [human being] by anyone anywhere can't be justified today."

Knight is the founder of the Voluntary Human Extinction Movement, an informal network of people dedicated to phasing out the human race in the interest of the health of the Earth. Knight, whose convictions led him to get a vasectomy in the 1970s, when he was 25, believes that the human race is inherently dangerous to the planet and inevitably creates an unsustainable situation.

"As long as there's one breeding couple," he says cheerfully, "we're in danger of being right back here again. Wherever humans live, not much else lives. It isn't that we're evil and want to kill everything -- it's just how we live."

Knight's position might sound extreme at first blush, but there's an undeniable logic to it: Human activities -- from development to travel, from farming to just turning on the lights at night -- are damaging the biosphere. More people means more damage. So if fewer people means less destruction, wouldn't no people at all be the best solution for the planet?

I've been thinking about this a lot lately because my wife and I have been talking about having a child. We're the kind of people who reduce, reuse and recycle. We try hard not to needlessly fritter away resources. We think globally and act locally in our day-to-day decisions. So while the biggest quandary of most couples in our shoes might be what color to paint the nursery, we have to ask ourselves, Is the impact of a new person justified?

The problem is stark: The United Nations estimates that the human population, currently at 6.5 billion, is well on its way to 9.1 billion in 2050. Many estimates place a sustainable population in which most of the people on Earth are able to enjoy their lives at between one and two billion.

By nearly every measure -- pollution, carbon emissions, forest loss, fishery depletion, soil fertility, water availability and others -- the growing population is wreaking havoc on the Earth's systems. And it's setting our civilization up for a big, hard fall.

As it is, even with my vegan diet, avid bicycling, recycling and energy-conservation measures, if everyone on the planet lived the way I do, we'd need three more Earths. As far as I know, they aren't making any more of these.

Meanwhile, almost 16,000 humans are born each hour. Regardless of the merits of reducing the population to nil -- as Knight advocates -- it's pretty clear that the world could do without any additional people.

Certainly without more Americans. In 1994, Charles Hall, an ecologist at SUNY Syracuse, performed a life-cycle analysis of the average American (PDF file) by determining each person's lifetime share of the nation's total consumption of various resources. It's the kind of study usually undertaken for assessing the impacts of a new product or policy, and the results are unsettling.

Hall and his colleagues found that a single new American born in the 1990s will be responsible, over his or her life, for 22 million pounds of liquid waste and 2.2 million pounds each of solid waste and atmospheric waste. He or she will have a lifetime consumption of 4,000 barrels of oil, 1.5 million pounds of minerals and 62,000 pounds of animal products that will entail the slaughter of 2,000 animals.

"In terms of energy usage alone, [which is] a convenient measure of environmental impact," Knight says, "the average Ethiopian uses one 310th of what we use. So when an American couple stops at two kids it's like an Ethiopian couple stopping at 620."

According to Knight, there are other ways people can have kids in their lives. "Adoption, foster-parenting, stepparenting -- there are a lot of opportunities for people who really do want to get involved with children." Knight himself is a substitute high school teacher in Portland, as befits his patient but forcefully clear demeanor.

Knight takes care to point out that VHEMT isn't anti-child. Many of its members are parents. Some of its members are children. In many ways, the idea of reducing the world's population is as much about human quality of life as it is about the health of the planet.

"May we live long and die out," says Naomi Thompson, quoting the VHEMT slogan. Thompson, who is in her late 20s and works as an analyst for Wells Fargo in San Francisco, has also concluded that childbearing is irresponsible. "It's not about wanting to kill people, but it's selfish to have a kid at this point when so many aren't getting the love and attention that they deserve."

"I really do love kids," she continues. (Thompson and Knight say they were raised in large, happy families.) "I know it might seem odd for someone who really likes kids to have this stance on breeding -- women are mothering, nurturing people, and I definitely have that in me. But women in this society feel a lot of pressure to have babies, and I would like to see more people expressing that by adopting instead."

The question of having children gets to the heart of some of our most basic drives, a place where rationality can take us only so far. Though I can picture myself as a father, I just can't see myself adopting.

I'm more like Mary and Mike Brune. The Alameda couple are longtime environmentalists. Mike Brune is executive director of the Rainforest Action Network, so he spends his entire workday thinking in excruciating detail about just how much trouble the planet is in.

Like most environmentalists -- even most Americans -- the Brunes have taken steps to reduce their environmental impact.

"We certainly do as much as we can to limit our consumption," says Mike Brune. "We made sure we live near mass transit. We have one of the new Priuses. We buy organic food almost exclusively. We feel that it's very important to connect our personal values to all aspects of how we live: where we work, what we eat, what we buy."

But when, after six and a half years of marriage, it came time for the couple to consider a child, those strong personal values came up against an even stronger drive.

"I understand rationally the argument for not having children -- I can see the point," says Mary Brune, a technical writer and, since becoming a mother, co-founder of Making Our Milk Safe, an organization that monitors industrial toxins in human milk (watch this space for more on that issue).

"I've talked to friends who have made certain that they can't have children so they don't bring another person into the world," she continues. "But for us there's a real primal need to have a child. For me, personally, I had a desire to bear my own child."

So they went for it: Their daughter Olivia is now 15 months old.

At RAN, Mike Brune works to transform some of the most powerful elements of our society, going after oil companies and banks to change the way they do business. He says that for him this kind of big-picture environmentalism doesn't translate to the personal decision of whether to have a child.

"The goal here isn't for Safeway to have one aisle of organic food -- it's to get to a point where all food is produced in a healthy way," he says. "The same would be true of hybrid cars: We don't want Ford Motor Co. to just have a few hybrid vehicles, we want to have every vehicle nonpolluting." For Mike Brune, the choice to have a child is a personal, emotional one that sits apart from the systemic change he's working for.

But does approaching the issue as an emotional question hinder our ability to address population problems? VHEMT's Knight says there's a taboo against talking about population control in what he calls our "natalist" culture -- a barrier that has resulted in many environmental groups either not addressing population or doing so inadequately.

"Nobody will come right out and say that this is unsustainable, you can't do this," says Knight. "If you really are serious about the environment and your impact, zero is the optimal number of offspring that we should be producing."

But the Brunes are sanguine. "We brought a new person into the world," says Mary Brune, "and we hope that she'll be one more soldier on the front lines who's going to fight for the Earth when she grows up."

Knight says even if little Olivia becomes the "firecracker radical activist" her father hopes, it's going to be extremely difficult for her to overcome the environmental original sin she embodies.

"I do think that if you added up a whole lifetime of one person, even living lightly," he says, "reproducing would bump you up into the Hummer-driver category." Rather than focus on raising new people in a certain way, says Knight, "if you instead help other people become the people that you think we all should be, you can have far more impact."

"In light of the number of species going extinct because of our increase, and the tens of thousands of children dying every day from preventable causes, there's just no good reason to have a child," adds Knight. "We have to ignore all those children to create another one. It's like saying, 'Well, they just don't matter.' But they do matter: They're all children in the human family."

But breeding is anything but rational, which is why I'm having such a hard time figuring out what to do. I'm a pessimistic optimist, or maybe an optimistic pessimist. But that kind of nuance doesn't wash when it comes to raising a kid -- either you do it or you don't. Nothing could be more hardwired: Every single one of our ancestors, dating back billions of years, has successfully reproduced -- it's the essence of what living things do.

It really comes down to whether you are an optimist about human nature. Having a kid is an implicit endorsement of the idea that it's possible to have a sustainable ecosystem that includes humans -- that it's possible to find a way out of the mess we've created.

Knight doesn't think people can do it.

"Other than a few examples of tribal societies, we never have lived sustainably," he says. "We're so dangerously clever that we can become very civilized and industrialized and separate ourselves from nature. Most of the people who do live close to nature are just a hand ax and a shotgun away from starting on the slippery slope that leads to driving around and talking on cell phones."

In many ways, it's an apples-and-oranges situation: The reasons not to breed -- stacked up next to the deep-seated biological and cultural satisfaction of having offspring -- can only illuminate the gulf between reason and emotion. It can't tell us which side of the gulf we'll spend the rest of our lives on -- only that we can't have it both ways.

I still have no idea what we're going to do. At least by thinking about it and entering into it consciously or not at all, we're rising above mere biology and taking a real step toward overcoming the animal drives that are consuming the planet -- a step both Knight and the Brunes agree is important in the evolution of the human animal.

"There's no end to the guilt you can feel as a parent, about everything as a person alive today," says Mary Brune. "But I'm grateful every day for being a mom and glad that she's here in our lives."

But, she adds, maybe one is enough: "It is something to question if we should have another child. Adopting a child instead is definitely something to consider."

Even Knight, in his oddly cheery brand of pessimism, thinks that the drive to breed may be insurmountable.

"It's not too likely that the Voluntary Human Extinction Movement is going to succeed," he told me. "I don't think any of us are so naive as to think that 6.5 billion people are going to say, 'Yeah, let's stop breeding, this is great.' But it's still the right thing to do."